It is known to lead a web of this nature around a rotating drum having a perforated peripheral wall through which drying air, possibly preheated, can be aspirated with the aid of a perforated exhaust pipe extending along the drum axis. The suction created along the drum periphery, aside from assisting in the penetration of the web, also helps hold the web in contact with the drum throughout the drying area. In order to facilitate the detachment of the partly or fully dried web from the drum periphery, it is further known to provide a stationary shield which hugs the inner surface of the drum wall along a zone of limited arc length to cut off the air flow through the interstices of this wall; see, for example, U.S. Pat. No. 3,112,054.
Since suction is generally applied to the exhaust pipe at one end only, the pressure differential across the peripheral drum wall varies along the axis. This results, especially with drums designed to accommodate wide webs, in a nonuniformity of the drying process along the web surface. For a more efficient mode of operation, moreover, a progressive intensification of this pressure differential in the direction of travel may be desirable.